Biaxially oriented polypropylene (BOPP) films used for packaging, decorative, and label applications often perform multiple functions. For example, in laminations they can provide printability, transparent or matte appearance, and/or slip properties. They can further be used to provide a surface suitable for receiving organic or inorganic coatings for gas and moisture barrier properties. They can also be used to provide a heat sealable layer for bag forming and sealing, or a layer that is suitable for receiving an adhesive either by coating or laminating.
However, in recent years, interest in “greener” packaging has been developing. Packaging materials based on biologically derived polymers are increasing due to concerns with renewable resources, raw materials, and greenhouse gases. Bio-based polymers are believed—once fully scaled-up—to help reduce reliance on petroleum, reduce production of greenhouse gases, and can be biodegradable. The biodegradable aspect is of interest to many snack manufacturers so as to provide litter abatement in addition to a lower carbon footprint package. Bio-based polymers such as polylactic acid (PLA)—which is currently derived from corn starch (but can be derived from other plant sugars) and thus, can be considered to be derived from a renewable or sustainable resource—is one of the more popular and commercially available materials available for packaging film applications. Other bio-based polymers such as polyhydroxyalkanoates (PHA) and particularly, polyhydroxybutyrate (PHB) are also of high interest.
Typically, BOPLA films are generally transparent with a high clarity and high gloss. In some packaging applications, this is very desirable for printing graphics with high visual appeal—or “pop”—and to provide high resolution of the images desired. PLA, being a polar polymer, naturally has a high surface energy which helps enable good wettability of the types of printing inks and colors used in packaging. However, there is also a desire in the packaging industry for a non-glossy, matte appearance film for certain graphic applications. A printed matte film typically gives a “paper-like” appearance to the package which has a certain aesthetic advantage and attractiveness for some applications.
Unfortunately, biaxially oriented PLA (BOPLA) film has been found to be limited to relatively low orientation rates when compared to BOPP manufacturing. This has an impact on productivity and cost. Because polylactic acid polymers are highly polar, it has been found that to effectively make BOPLA films, orientation rates typically found with biaxially oriented polyester (OPET) films must be used, e.g. roughly 3× in the machine direction (MD) and 3× in the transverse direction (TD). If BOPLA films are oriented in the transverse direction higher than a nominal 3 or 4×, film breaks are prone to occur and production of BOPLA films cannot be achieved. However, since BOPLA has been targeted to potentially replace BOPP in packaging, the potential cost of BOPLA due to this reduced productivity may be too high allow for BOPLA to effectively replace BOPP.
BOPP film manufacturing typically has a MD orientation rate of 4-5× and a TD orientation rate of 8-10×. Thus, BOPP films are produced much wider than BOPLA films and have a higher production output and customer width programming. Couple this lower productivity with the current price of PLA resin being higher than polypropylene resin, and BOPLA films suffer a severe cost disadvantage.
For BOPP films, matte appearance is generally achieved by using a resin mixture of incompatible resins. Popular blends commercially available generally use a blend of propylene homopolymer with ethylene homopolymer, with low density polyethylene, medium density polyethylene, or high density polyethylene being used. Also commercially available and used are block copolymers of ethylene and propylene which can give a matte appearance. Alternatively, there are also mineral fillers that can be used to give a matte appearance to the film, such as fine talcs or clays
U.S. Pat. No. 7,128,969 describes a film composed of a base layer of PLA with a minority component of a thermoplastic or polyolefin such as polypropylene or polyethylene, typically less than 1% by weight of the base layer. Such a formulation is particularly suitable for thermoforming or biaxial stretching by means of pneumatic drawing or other mechanical forming. However, the formulation is not suitable for high transverse orientation rates in excess of 6 TDX; the highest TDX cited in the examples is 5.5. In addition, the small amount of polyolefin used as a drawing aid in the PLA base layer is not enough to produce a suitably consistent matte appearance.
EP Patent No. 01385899 describes a multi-layer film design using a PLA base layer formulated with a cyclic polyolefin copolymer (COC) as a cavitating agent to produce an opaque biaxially oriented PLA film. However, this invention uses a cavitating agent that can be costly to use.
U.S. patent application Ser. No. 12/333,047, the entire contents of which is herein incorporated by reference, describes the use of ethylene-methacrylate copolymers as a process aid to increase significantly the transverse orientation rate of biaxially oriented PLA films. However, the resulting films are transparent, not matte or opaque.